1. Field of the Invention
One embodiment of the present invention relates to a photodetector circuit. Further, one embodiment of the present invention relates' to an input device. Further, one embodiment of the present invention relates to an input/output device.
2. Description of the Related Art
In recent years, technological development of an input device which inputs data with the use of a photodetector circuit (also referred to as an optical sensor) capable of generating data having a value corresponding to the illuminance of incident light, an input/output device which inputs data with the use of the photodetector circuit and outputs data in accordance with the input data, and the like has been advanced.
As an example of the input device, an image sensor can be given. As examples of the image sensor; a CCD (charge coupled device) image sensor, a CMOS (complementary metal oxide semiconductor) image sensor, and the like can be given (e.g., Patent Document 1).
As an example of the input/output device, an input/output device which includes a plurality of display circuits and a plurality of photodetector circuits in a pixel portion and has a function of detecting coordinates of an object to be detected which overlaps with the pixel portion (also referred to as a coordinate detecting function) by detecting light incident on the photodetector circuits and a function of generating image data of the object to be detected (also referred to as a reading function) can be given (e.g., Patent Document 2). For example, with the coordinate detecting function, the input/output device can also function as a touch panel. Further, with the reading function, the input/output device can also function as a scanner and can display an image based on image data generated with the reading function by using the plurality of display circuits.
The photodetector circuit in the above input device or the above input/output device includes a photoelectric conversion element, a charge accumulation control transistor, and an amplifier transistor. In this case, when the charge accumulation control transistor is turned on, the voltage of a gate of the amplifier transistor is changed in accordance with the illuminance of light incident on the photoelectric conversion element. Then, the charge accumulation control transistor is turned off, whereby the changed voltage of the gate of the amplifier transistor is held as data, and voltage having a value corresponding to current that flows between a source and a drain of the amplifier transistor in accordance with the held voltage of the gate of the amplifier transistor is output as output data.